Chromatography has been employed as a means of analyzing mixtures of hydrocarbons and other compounds found in petroleum oil.
In Petroanalysis 81, Chapter 9, it is disclosed that a hydrocarbon mixture combined with a solvent results in an eluate being recovered from the exit end of a chromatographic column which comprises the following types of molecular species, in order, namely: saturates (e.g. paraffins and naphthenes), olefins and aromatics. The remaining molecular species, which are (in general) polar compounds, have a relatively high affinity for the solid chromatographic material and can only be recovered in a reasonable time and reasonably completely by interrupting the flow of the solvent and substituting therefor a different solvent having a relatively high affinity for, e.g., heteroaromatic compounds. The said different solvent is passed through the column in a direction which is opposite to that of the first solvent so that after a reasonable time interval of this so-called back-flush, polar compounds (resins) are present in the back-flush eluate. The change in solvent from the first solvent, pentane, to the second solvent, methyl t-butylether, necessitates the use of two different eluent detectors, i.e. one using refractive index and the other using ultra-violet absorbance at 300 nm.
Backflushing is inconvenient because of the mechanical complications involved in automating a chromatography unit utilizing it and also because it, in itself, does not ensure complete recovery of chemical species which are strongly adsorbed by the stationary phase of the column.
In Journal of Liquid Chromatography, 3(2), 229-242 (1980), a hydrocarbon mixture containing asphaltenes is subjected to chromatographic analysis only after mixing the mixture with hexane to precipitate asphaltenes which are separated by filtration and then determined gravimetrically. The hexane solution of the remaining hydrocarbons is then passed through a column of particles of u-Bondapak-NH.sub.2 where it separates into an eluent comprising, initially, saturates and then aromatics, as determined by the refractive index of the eluent. Resin which is retained on the column is backflushed off the column and determined by difference. The separation quality of the column is maintained by flushing it with a solution of 1/1 methylene chloride/acetone after every 20 samples and then regenerating with methylene chloride and hexane for repeatable retention times. Changes in the refractive index of the eluents, indicative of the presence of respective chemical species, are monitored and correlated with absolute amounts of the chemical species by means of a Hewlett-Packard 3354B computer using the so-called "Zero" type method The drawbacks of this technique are that: (1) asphaltenes are determined gravimetrically rather than by chromatography so that the technique does not lend itself readily to automatic operation; (2) backflushing is employed, and not all the material in the sample fed to the column is recovered in the eluent as is evidenced by the stated need to clean the column periodically (further reference to this significant problem will be made hereinafter); (3) the refractive index detector which is employed has a response which varies with each type of molecular group such as paraffins, naphthenes, aromatics and resins, and therefore cannot provide a universal response for a given mass which is independent of the nature of the sample of the feedstock which is being analyzed.
In Journal of Chromatography, 206 (1981) 289-300, Bollet et al., a rapid high-performance liquid chromatography technique for separating heavy petroleum products into saturated, aromatic and polar compounds is described. A column containing a stationary phase of silica bonded NH.sub.2 ("Lichrosorb NH.sub.2 ") is used. Two chromatographic analyses are needed in order to determine the composition of a sample. In the first analysis, saturated compounds are separated from aromatic and polar compounds, using hexane or cyclohexane as the mobile phase. In the second analysis, saturated and aromatic compounds are separated from polar compounds using 85 vol % cyclohexane, 15 vol % chloroform as the mobile phase. The eluents are monitored by differential refractometry for saturated, aromatic and polar compounds, and by ultraviolet photometry for polar compounds. The proportions of saturated and polar compounds are said to be determinable by these monitoring techniques and the proportion of aromatic compounds found by difference. However, the method described, in common with all other reports of high performance liquid chromatography (HPLC) for analysis of samples of heavy hydrocarbon oil mixtures, is limited by the lack of a means and method for quantitative and feedstock-independent detection and monitoring. Thus, for both refractive index (RI) and ultraviolet (UV) detectors, "response factors" must be derived by separating samples of the feedstock on a larger scale, known in the art as "semi-preparative liquid chromatography" and then gravimetrically weighing the recovered analyte (after removal of the solvent(s) added to the sample for the purpose of the chromatographic separation). Response factors are dependent on the nature of the feedstock and its boiling range, and it is therefore essential to perform the relatively large-scale separation to obtain accurate results with the HPLC analysis. Thus, the potential benefits of speed and increased resolution which should be possible with HPLC have not heretofore been fully realized in practice. The technique of Bollet et al is compared with the method of the invention in the Comparative Example given hereinafter.
Reference is also made to Klevens and Platt, J. Chem. Phys. 17:470 (1949). Similarities are reported in the total oscillator strength for-electronic transitions of cata-condensed aromatics. However, the article does not relate to HPLC analysis, nor does it recognize or suggest that a UV detector operating in a specific wavelength range can be used in HPLC to derive an integrated oscillator strength output which quantifies the aromatic carbon in petroleum and shale oil feedstocks. Furthermore, cata-condensed aromatics constitute only a minor fraction of the various aromatics in a hydrocarbon feedstock such as petroleum. Other aromatics, including para-condensed aromatics, alkyl aromatics, and thiophenic aromatics which behave spectroscopically differently from cata-condensed aromatics, are also generally present in a hydrocarbon feedstock.